EME Clear Key: Run Session Closed algorithm before resolving returned promise

BUG=659374
TEST= https://www.w3c-test.org/encrypted-media/clearkey-mp4-playback-temporary-events.html after fix for https://github.com/w3c/web-platform-tests/issues/4058

Review-Url: https://codereview.chromium.org/2446993003
Cr-Commit-Position: refs/heads/master@{#427755}
2 files changed